The present invention generally relates to polymeric composites and more particularly, to a method of making an electrically conductive polymeric composite for use in an electric heating element or the like.
It is generally known that an electrically conductive polymeric composite is produced by dispersing fine powder of an electrically conductive material such as carbon black, metal powder, metal fiber, etc. in a polymeric material. Such electrically conductive polymeric composite is widely used as an antistatic material or an electric heating element. In the known electrically conductive polymeric composite used as an electric heating element, carbon powder dispersed in a polymer and the polymer are required to be maintained at a proper percentage composition and the resistance value of the composite should be stabilized for a long time. Therefore, the known electrically conductive polymeric composite has been disadvantageous in that the polymer and the carbon black are inevitably required to be examined carefully in quality so as to be combined with each other. However, even if the polymer and the carbon black are examined thoroughly in respect to the quality thereof, the carbon black is merely dispersed uniformly in the polymer based on the affinity between the polymer and the carbon black. Accordingly, when the known electrically conductive polymeric composite has been used for the long time, a transmission passage of an electric current is displaced due to the cohesion of the carbon black, etc. and thus, it is difficult to prevent variations of the resistance value. Furthermore, in the case where the known electrically conductive polymeric composite is used for a special purpose and therefore, is, for example, required to have a positive temperature coefficient of resistance, such a case may arise that a polymer which is most unsuitable for stabilizing the resistance value, for example, polyethylene should be employed. In the case of a thermoplastic having a high crystallinity such as polyethylene, since a mechanism for fixing the carbon black dispersed in the thermoplastic scarcely works owing partly to a fact that such thermoplastic has almost no functional group, it is usually inevitably necessary to perform a crosslinking process such as irradiation by electron rays, etc.
As prior art in this field, U.S. Pat. No. 3,243,753 discloses a basic technology in which a cross-linking process is not performed, while U.S. Pat. No. 3,673,121 proposes an annealing process for stabilizing the resistance value. Furthermore, U.S. Pat. No. 3,861,029 discloses a crosslinking process which is performed by irradiation with electron rays after an annealing process. If the process disclosed in the above described prior art documents are employed, it is possible to produce a stable electrically conductive polymeric composite even when polyethylene is used as its polymer. However, since the crosslinking process is performed by irradiation with electron rays at a final step in this case, it is extremely difficult to perform the crosslinking process of the prior art electrically conductive polymeric composite, thereby constituting a serious obstacle to formation of an arrangement in which a layer having another function, for example, a temperature sensitive layer is integrally formed with the prior art electrically conductive polymeric composite. Meanwhile, as disclosed in U.S. Pat. No. 4,200,973, the production of an electrically conductive polymeric composite according to prior art processes are so inconvenient that an annealing process must again be performed after the crosslinking process, thus making the process steps complicated.